Sun Sep 7 2008

Left conical bend

I started this evening working on drilling the F-728A and F-729A to the F-778 aft bottom skin. While under the canoe, I noticed that the distance from where the AN470AD4 rivets attach the F-727L & R baggage floor ribs to the F-706B bulkhead to the edge of F-728A and F-729A looks too small for the head of the AN470AD4 rivet.

I went ahead and match drilled the F-706B bulkhead to the F-727L & R baggage floor ribs so I could fit an AN470AD4 rivet in to check the head clearance.

Sure enough, the rivet head overlaps the flanges of the F-728A and F-729A slightly.

...so I marked around the head with a sharpie to give me a "file-to" line at each of the rivet locations.

After taking a break for dinner, I remembered to bring this little gem down from the kitchen table. It's a check valve for the brake fluid reservoir to prevent dripping while inverted. I ordered mine from Alex Peterson's website, Peterson Innovation.

Anyway, I filed the notches in the F-728A and F-729A. Actually, I also filed them in the F-730 doubler plate as well since that plate will be in position just aft of the F-706B.

Then I decided to work on the conical bend of the F-770 side skins. The skins come with a roughly 1/8 inch bend relief hole. That means that the max bend radius I could use is 1/16 inch.

...however, when I check my aircraft sheetmetal book, the minimum recommended radius for a T3 temper is 3X the material thickness or .075 in this case. I've seen on some other builders websites where having the bend be too sharp results in a crack. I didn't want to have to deal with that...

...so I decided to elongate the relief hole somewhat to accomodate a larger bend radius.

I cut a couple pieces of angle to clamp the side skin to the workbench and also to use as lever. After drilling all the holes and installing clecos in every hole, I was ready to try the bend.

Oh, one other important point...I filed a 3/32 radius along the edge of the angle which was used to clamp the side skin to the work bench. Hopefully that will be enough to prevent a crack from forming.

Here's the conical skin after the initial bend. It went pretty smoothly except for a couple things. I had to flip a few of the clecos around to the bottom side of the angle near the apex of the cone. I also had difficulty getting the gradual bend at the aft end of the skin to be uniform. It kind of wanted to kink at the rivet holes. A little hand work resolved that problem though.

I clecoed the skin back in place. It looks pretty good.

It looks like my bend at the vertex isn't quite sharp enough. It's lifting up the F-776 center bottom skin slightly.

I also have an issue with the holes not lining up correctly near the forward end of the bend. If I put clecos in these holes, they tend to hold the side skin away from the side of the F-623L by about 1/16 inch.

After tweaking the bend some more, it fit better. Now the F-776 center bottom skin lays down nice and flat.

I've still got the problem with the holes lining up, but I'm pretty sure I can use oops rivets or AN426AD4 rivets here instead of AN426AD3 rivets.